For a very basic firmware, meant for validating your qmk build environment as much as anything else, start here https://github.com/qmk/qmk_firmware/tre ... red/onekey
You can have a look and see how the proton_c config differs from the teensy_2 config, for example.
If you get that to work okay, you could maybe go to https://kbfirmware.com and set up the matrix *as though* you were going to flash it to a teensy, then on the last tab select Download Source Files. This will give you a set of files with the rows and columns configured correctly for a teensy -- then you can use what you learned above to adjust the config for the proton_c and its appropriate pins. Someone else might have an easier way though.